A conventional electric bicycle uses a motor to drive the sprocket mounted to the crank axle and a chain connected between the sprocket and the rear wheel sprocket so that when the sprocket is rotated by the motor, the rear sprocket is rotated by the chain and the bicycle moves forward. Although the motor provides the mechanical force to assist the rider to power the bicycle, the rider's feet on the pedals of the crank have to rotate with the rotation of the crank. This is inconvenient for some older riders, and when the rotational speed of the crank is raised, the rider will feel uncomfortable and unsafe because they feel that the bicycle cannot be well controlled by them. Furthermore, the rider's feet rotating with the crank makes the rider feel tired within a short period of time of operation.
The present invention intends to provide an electric drive device for a bicycle wherein the crank will not rotate with the rotatable tube when the bicycle is powered by the motor so that the shortcoming mentioned above can be resolved.